- Make the top part of the tree a little more fatter and fuzzier- Make the trunk of the tree a little skinnier- Add some light source ( Like what DarkManor said, a light source would make it look nicer )- It would be nice if the leaves pop out more

E.x.http://browse.deviantart.com/?qh=&section=&global=1&q=tree+pixel&offset=24#/d3grg8o ( Not made by me )